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    "给定一个二叉树，返回其节点值的锯齿形层次遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。\n",
    "\n",
    "例如：\n",
    "```\n",
    "给定二叉树 [3,9,20,null,null,15,7],\n",
    "\n",
    "    3\n",
    "   / \\\n",
    "  9  20\n",
    "    /  \\\n",
    "   15   7\n",
    "返回锯齿形层次遍历如下：\n",
    "\n",
    "[\n",
    "  [3],\n",
    "  [20,9],\n",
    "  [15,7]\n",
    "]\n",
    "```\n",
    "来源：力扣（LeetCode）\n",
    "链接：https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal\n",
    "著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。"
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